Without it being more explicit, and not knowing the franchise, I assume that fan letter was from one of her friends who is also an idol?

last edited at Nov 7, 2016 3:51AM

im pretty sure thats what happened, it was one of the 3 at the end

Yes, the middle of the three, the little goth/horror girl (probably the little skull sticker on the envelope would have been enough to tip off fans if Ryo hadn't said "Koume").